The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a contact plug which increases contact area.
Plugs are formed to connect metal interconnection lines on different layers. The tungsten plug process has been commonly used to form the plugs.
In the tungsten plug process, a tungsten layer is formed inside contact holes to form tungsten plugs. Then, an etch back process using a dry etching method is performed to isolate the plugs from each other.
FIGS. 1A and 1B illustrate a typical method for fabricating a semiconductor device. As shown in FIG. 1A, an inter-layer insulation layer 12 is formed over a substrate 11. The inter-layer insulation layer 12 is etched using a hard mask pattern 13 to form a contact hole 14.
As shown in FIG. 1B, the hard mask pattern 13 is removed. A tungsten layer is formed over the entire surface until the contact hole 14 is filled. Then, an etch back process is performed to form a tungsten plug 15. A metal layer is formed over the tungsten plug 15 and then patterned to form a metal line 16.
As the semiconductor device becomes highly integrated, the depth of the contact hole 14 gets deeper and thus, the deep contact hole is formed. After the tungsten plug 15 is formed and etched, an over etch is also performed to achieve etch uniformity throughout the wafer.
The over etch may generate plug loss on the tungsten plug 15 just inside the contact hole 14. A reference letter L1 shows the plug loss. The plug loss L1 may be great in certain parts of the wafer.
FIG. 2 is a micrograph illustrating the plug loss observed when the conventional method is employed. A large amount of the plug loss is generated on the top surface of the tungsten plug. Since a metal layer including aluminum (Al), copper (Cu), or tungsten (W) is formed over the tungsten plug, a plug loss can cause the contact area between a metal line and the tungsten plug to be reduced. As a result, there is often a loss in current from the increased resistance. Furthermore, the top surface of the tungsten plug is lower than the surrounding surface of the inter-layer insulation layer. Accordingly, if particles and various pollutants of a subsequent process flow into the tungsten plug gap, the tungsten plug may not be connected to the metal line.